In order to vary an output torque according to need, or to control a rotational speed of the prime mover to an efficient speed, the power transmission unit of this kind has been used widely in the prior art. Specifically, a mechanism comprising a plurality of gear pairs is used in general to vary the torque and the rotational speed. In case of using the gear pairs to vary the torque and the rotational speed, power transmission efficiency can be improved in comparison with that in the case of using a belt, a chain or a fluid.
For example, Japanese Patent Laid-Open No. 2002-204504 discloses a power transmitting system composed mainly of a twin-clutch type transmission. The system disclosed in Japanese Patent Laid-Open No. 2002-204504 comprises: two clutch shafts connected selectively with an engine by a clutch; a plurality of gear pairs arranged between the clutch shafts and an output shaft; and a clutch mechanism for connecting the gear pairs selectively with the clutch shaft or with the output shaft. A differential motor generator having a rotor and a stator both of which are rotatable are arranged between those clutch shafts, and connected with each clutch shaft through a bevel gear. That is, the motor generator is arranged perpendicular to the clutch shafts.
On the other hand, Japanese Patent Laid-Open No. 2004-293795 discloses a transmission system, which is adapted to distribute a torque of an engine to two clutch shafts through a differential type motor generator, and to output the torque from the clutch shafts to an output shaft through gear pairs individually set to a predetermined gear ratio.
According to the unit or the system taught by the above-mentioned patent documents, the differential action is achieved between the engine and the clutch shaft or the gear pair. Therefore, the rotational speed of the gear pair attached to one of the clutch shafts can be controlled by the motor generator while transmitting the torque to the output shaft through the other clutch shaft. That is, according to the unit or the system taught by the above-mentioned patent documents, the rotational speed of the gear pair to be used to transmit the torque can be synchronized with the rotational speed thereof before the speed change operation. Therefore, a shift shock can be prevented or minimized, and the output shaft torque can be maintained even during the speed change operation.
However, according to the unit or the system taught by the above-mentioned patent documents, the engine torque is distributed selectively to the two clutch shafts, and the differential type motor generator is arranged between the clutch shafts perpendicular to those clutch shafts through the bevel gear. Therefore, the number of shafts has to be increased and clearances among the shafts have to be widened. For this reason, the power transmission unit is enlarged entirely and a weight thereof is increased.